Optical methods for control of critical dimensions and/or profile of etched and lithographic structures in high-volume semiconductor manufacturing environments are gaining wide acceptance, largely due to the promise of rapid, nondestructive real-time feedback for cost-effective process control.
Among the earliest current art metrology systems are scatterometry systems, such as the method taught in U.S. Pat. Nos. 5,164,790 or 5,703,692, which determine angle-resolved spectral response from periodic structures. Later current art metrology systems employed traditional thin film analysis tools, such as broad-band reflectometers and ellipsometers, as taught in U.S. Pat. Nos. 5,432,607, 6,281,674, or 6,898,537.
Most of the various current designs operate in a spectral region between deep ultraviolet (DUV) (˜200 nm) and near infra-red (˜1000 nm) wavelengths. This limits the fundamental resolution of such systems when measuring structures much smaller than the incident wavelength, and causes the metrology to lose sensitivity to the details of profile shape. As such, current optical metrology becomes increasingly obsolete as semiconductor device dimensions shrink.
At a given wavelength range, the more incident conditions an optical tool measures, the greater the sensitivity of the measurement to a greater number of parameters. Accordingly, some recent current art systems overcome some of the resolution issue by combining ellipsometric and polarimetric (polarized reflectance) data, such as the methods taught in U.S. Pat. Nos. 6,713,753 and 6,590,656, at the expense of greater complexity and less versatility in a manufacturing environment. Another approach combines broadband reflectance, polarimetric, or ellipsometric data with multiple angle of incidence measurements, such as the method taught in the article T. Novikova, A. Martino, S. B. Hatit, and B. Drevillon, “Application of Mueller polarimetry in conical diffraction for critical dimension measurements in microelectronics”, Appl. Opt., Vol. 45, No. 16, p. 2006. Such systems are complicated to operate, often slow, and are very hard to integrate into the manufacturing process. Aside from this, there is still the fundamental issue that resolution information is lost as the measured feature sizes decrease, and after a certain point, no amount of additional datasets will compensate for this.
On another front, optical data from metrology tools are often analyzed using rigorous solutions to the boundary value problem. One of the most common analysis technique for periodic structures is the rigorous coupled wave (RCW) method, which is sometimes also referred to as the Fourier Modal method. The RCW method is used to compute theoretical optical spectra representative of the structure being measured as the model parameters are changed during a regression analysis. The optimized parameters are the measurement result.
The RCW calculation can be very computationally intensive. In some cases, a library database is used to store pre-generated spectra to be compared with the measured spectra during measurement. Even then, the efficiency of the calculation is important since hundreds of thousands or even millions of spectra can be required for the database.
The special case of normal incidence benefits from symmetry conditions at all wavelength ranges, allowing for the most efficient RCW calculations. In addition, a normal incidence reflectometer is more suited to integration into the device manufacturing process, being less complicated to operate, easier to maintain, and more compact than the angle-resolved or ellipsometric solutions mentioned above.
Thus, it is desirable to have a reflectometer configured for normal incidence measurement for practical reasons, but also capable of using below deep ultra-violet (DUV) wavelength light for enhanced measurement capabilities. Instances of normal incidence polarized reflectometry in the current art, such as the one disclosed in U.S. Pat. No. 6,898,537, are not suitable for operation below DUV. The patent teaches a calibration method to account for the offset between different polarization conditions, which will not work in the region below DUV due to contaminant buildup during the tool's operation. In general, it is quite difficult to polarize light below ˜160 nm. In addition, the calibration of the absolute reflectance used by the system disclosed in U.S. Pat. No. 6,898,537 is complicated by the lack of reliable reflectance reference standards in the range below DUV. Therefore, the method disclosed in U.S. Pat. No. 6,898,537 is unsuitable for work below DUV wavelength range. A further complication arises in the use of polarized reflectance with an r-θ stage, and an elaborate polarization alignment procedure is required during measurement of periodic structures, since the orientation of the structures will vary as a function of r-θ position.